CHAPTER 3: VEGETATION GRADIENTS AROUND OPEN AND CLOSED WATERHOLES IN THE KRUGER NATIONAL PARK
3.3 RESULTS
Herbaceous species composition
Clear piosphere patterns were found for herbaceous species composition in both 1990 and 2006, for most sites (Fig. 3.1 and 3.2). The biplots shown are from the Marheya waterhole in 1990 and 2006. The patterns found at the 11 waterholes were very similar, and these two examples have been chosen as representative of the general pattern in the two periods. In 1990, plots close to the waterholes (0 – 50 m) were typified by high abundances of forbs, Tragus berteronianus, and Eragrostis spp. At intermediate distances from water (50 – 150 m), Dactyloctenium aegyptium, Urochloa mosambicensis and Chloris virgata were common. The plots 500 – 5000 m from water tended to be associated with palatable, grazing-sensitive species such as Themeda triandra and Digitaria eriantha. Only the first two of these zones present in 1990 were represented in 2006.
Fig. 3.1. First and second axes of a detrended correspondence analysis (DCA) of herbaceous species composition data along a distance
eigenvalues for DC axes 1 and 2 were 0.371 and 0.095, respectively. The two axes together accounted for 52.0 % of the variation in species composition. Species abbreviations are tabulated (Table 3.2) in Appendix A. Circles represent sites; triangles represent species. Sites with sma numbers are close to the waterhole, those with larger numbers are further from the waterhole (1
= 1 – 10 m, 2 = 10 - 20 m, 3 = 20 – 510 m, 8 = 1000 – 1010 m, 9 = 2000
First and second axes of a detrended correspondence analysis (DCA) of herbaceous species composition data along a distance-from-water gradient at Marheya waterhole (1990). The nd 2 were 0.371 and 0.095, respectively. The two axes together accounted for 52.0 % of the variation in species composition. Species abbreviations are tabulated
Circles represent sites; triangles represent species. Sites with sma numbers are close to the waterhole, those with larger numbers are further from the waterhole (1
, 3 = 20 – 30 m, 4 = 30 – 40 m, 5 = 50 – 60 m, 6 = 60 1010 m, 9 = 2000 – 2010 m, 10 = 5000 – 5010 m).
26 First and second axes of a detrended correspondence analysis (DCA) of herbaceous eya waterhole (1990). The nd 2 were 0.371 and 0.095, respectively. The two axes together accounted for 52.0 % of the variation in species composition. Species abbreviations are tabulated Circles represent sites; triangles represent species. Sites with small numbers are close to the waterhole, those with larger numbers are further from the waterhole (1 60 m, 6 = 60 – 70 m, 7 = 500
Fig. 3.2. First and second axes of a detrended correspondence analysis (DCA) of herbaceous species composition data along a distance
eigenvalues for DC axes 1 and 2 account were 0.823 and 0.09
together accounted for 52.6 % of the variation in species composition. Species abbreviations are tabulated (Table 3.2) in Appendix A.
with small numbers are close to waterhole (1 = 1 – 10 m, 2 = 10 70 m, 7 = 500 – 510 m, 8 = 1000
. First and second axes of a detrended correspondence analysis (DCA) of herbaceous species composition data along a distance-from-water gradient at Marheya waterhole (
values for DC axes 1 and 2 account were 0.823 and 0.093, respectively. The two axes together accounted for 52.6 % of the variation in species composition. Species abbreviations are tabulated (Table 3.2) in Appendix A. Circles represent sites; triangles represent species. Sites with small numbers are close to the waterhole, those with larger numbers are further from the
10 m, 2 = 10 - 20 m, 3 = 20 – 30 m, 4 = 30 – 40 m, 5 = 50 510 m, 8 = 1000 – 1010 m, 9 = 2000 – 2010 m, 10 = 5000 – 5010 m).
27 . First and second axes of a detrended correspondence analysis (DCA) of herbaceous eya waterhole (2006). The 3, respectively. The two axes together accounted for 52.6 % of the variation in species composition. Species abbreviations are Circles represent sites; triangles represent species. Sites the waterhole, those with larger numbers are further from the 40 m, 5 = 50 – 60 m, 6 = 60 –
5010 m).
28 Distance from water had a significant effect on species composition (from CCA) at all sites (range in F: 3.004 to 8.842, range in p: 0.002 to 0.032), and effect of time and the interaction of distance with time were also significant at all sites (range in p: 0.002 to 0.054). This result was confirmed by the blocked MRPP (Table 3.1, Appendix A), for which the T-statistic (a measure of the difference between two groups) was significant (range in p: 0.00003 to 0.02538) for all sites, although a large range was obtained (Range in T: -2.47 to -10.60). Larger compositional changes occurred between 1990 and 2006 at open sites (mean T = -7.17) than at closed sites (mean T = -5.66).
All three closed sites became more homogenous in species composition (mean within-block ED) between 1990 and 2006, with a mean decline of 5.89 ± 4.20 in ED (measure of internal heterogeneity at a site). Four open sites gained heterogeneity, and two became more homogenous between 1990 and 2006. Sites that were open in 2006 were more heterogeneous in species composition than sites that were closed, in both 1990 (when all sites were open) and 2006 (Table 3.1, Appendix A). Thus, heterogeneity was related more to site (waterhole location) than to waterhole status per se. The patterns of change (where along the distance gradient compositional change occurred) along the piosphere gradients (ED between transects/plots at a site) were highly site-specific – there was no consistent response to distance from water.
At six sites, forbs declined in their relative contributions to species composition from 1990 to 2006, whilst increaser species (pioneer and sub-climax grass species that increase in abundance in heavily/overgrazed savanna or grassland, called Increaser II species by (Dyksterhuis 1949)) increased and decreasers (palatable climax grass species that decrease in abundance in over- or under-grazed grassland) declined (Fig. 3.3). The plot shown (Fig. 3.3) is from the Witpens waterhole, selected because it is representative of this general pattern. Most sites experienced a large, significant overall increase in the relative abundance of Urochloa mosambicensis (range in t: 1.479 to 7.213, range in p: 0.001 to 0.021). Changes in this species were the foundation for changes in the increaser II group described above. Other important species in this functional group where Dactyloctenium aegyptium, Eragrostis trichophora and E. superba, which increased at most sites from 1990 to 2006. Temporal changes to the decreaser group were mostly determined by changes in Digitaria eriantha, Themeda triandra, Panicum coloratum and Brachiaria nigropedata.
29 Fig. 3.3. The change in the contribution (%) of different plant functional groups between 1990 and 2006 (at the Witpens waterhole). Values > 0 indicate a relative gain in a particular plant functional group between 1990 and 2006; values < 0 indicate a loss.
There were generally large increases in the proportion of plot-pairs with low (< 0.25) Bray- Curtis dissimilarity values, from 1990 to 2006. These losses of compositional heterogeneity were largely accounted for by plots close to the waterholes (0 – 150 m). There were, however, also increases in the proportions of plot-pairs with high (> 0.75) Bray-Curtis dissimilarity values, accounted for by comparisons of plots distant (> 500 m) from water with those close to water (<
150 m). Thus, between 1990 and 2006, there was a loss of compositional heterogeneity close to the waterholes, but areas close to water and areas distant from water diverged. Plots further than 500 m from water also tended to have high Bray-Curtis values when compared with each other.
Thus, compositional heterogeneity increased as distance from water increased. There were no consistent patterns at closed or open waterholes with respect to compositional heterogeneity.
There were no distinct patterns in species richness and diversity associated with open or closed sites, and the responses of both indices to distance from water were highly variable, across sites and time periods. Four sites had an increase in Shannon’s diversity from 1990 to 2006, and five sites stayed the same or decreased. This trend was mirrored by the equitability values. There was thus no clear piosphere pattern with respect to species diversity.
-120 -80 -40 0 40 80 120
% Contribution (2006 -1990)
Log10Distance from Water (m)
FORBS INCR 2 DECR
30 Basal cover & biomass (herbaceous)
Herbaceous biomass and basal cover both responded very variably to distance from water in 2006. In 1990, both basal cover and biomass were lowest in 20 – 100 m ‘sacrifice’ zones close to the waterholes and increased with distance from water beyond that, creating three-zoned piospheres. There were no significant differences in herbaceous basal cover between open and closed sites in 2006 (t =1.10, df = 8.12, p = 0.30) nor between the same site groups in 1990 (t = 0.14, df = 6.62, p = 0.89), but there were significant increases in basal cover from 1990 to 2006 (mean1990 = 0.77 ± 1.09 %, mean2006 = 8.57 ± 6.46 %, t = 12.369, df = 100, p < 0.001). We note that mean annual rainfall in 1990 (MAP 501 mm) was lower than in 2006 (MAP 635 mm), which may have caused the higher basal cover in 2006 (Fig. 3.4). Herbaceous standing crop was also not significantly different between open and closed sites in 2006 (t = 1.01, df = 6.00, p = 0.35), nor between the same two groups of sites in 1990, when all waterholes were open (t = 0.10, df = 2.59, p = 0.92). Herbaceous standing crop was higher also significantly higher (t = 13.131, df = 68, p < 0.001) in 2006 (mean = 4 746.47 kg.ha-1) than in 1990 (mean = 957.22 kg.ha-1).
31 Fig. 3.4. The change in annual wet season (July – June) rainfall (solid line) and five-year running mean rainfall (dashed line), as a % of the long-term average, between 1985 and 2006. The two study periods (1989-1990 and 2005-2006) are indicated by circles.
Woody composition and structure
We found no significant effect of waterhole closure on overall woody plant density, nor for any specific tree height class (range in t: 0.38 to 1.27, range in df: 4.35 to 6.93, range in p: 0.26 to 0.80). In addition, there were no consistent patterns along the distance from water gradient for any tree height class.
At most sites, 20 – 40 % of the comparisons between distance classes yielded low Bray- Curtis dissimilarity values (< 0.25) for woody community structure, but all 11 sites also had at least 20 % of comparisons with high Bray-Curtis values (> 0.75). Thus, there was an almost binomial pattern in heterogeneity – almost half the comparisons between distance classes at each site had very low heterogeneity, and the remainder very high heterogeneity. Unlike the comparisons made for herbaceous species composition, there were no piosphere patterns for
32 heterogeneity – transects with particular tree height distributions occurred along the entire distance gradient.
Distance from water did not have a significant effect (p > 0.1) on woody community composition at the majority of sites (n = 8). The only species for which a general trend was noticeable was Dichrostachys cinerea. This species was most abundant at intermediate distances (200 – 500 m) from water.
The analysis of species compositional dissimilarity between all pairs of transects at each piosphere revealed that six sites did not have any pairs with Bray-Curtis values below the homogeneity threshold value of 0.25, and of the remaining five sites, only three had more than 10 % of pairs below this threshold. For the heterogeneity threshold value (Bray-Curtis > 0.75), only two sites had less than 50 % of pairs that exceeded the threshold. Three sites had more than 80 % of pairs with Bray-Curtis values of more than 0.75. Therefore, overall, there was high compositional heterogeneity in the woody community at the waterholes surveyed. Shannon’s diversity (H’) showed no distinct piosphere pattern at open sites (constant diversity along gradient), although diversity increased with distance at three closed sites. Equitability (J) followed a similar trend to diversity (H’) along the distance gradient at most sites.